1. Field of the Invention
The present invention relates to an RF tag system having commands for reading and writing data. More particularly, the present invention relates to an RF tag system having group commands that reduce the number of steps needed to read data from a tag or write data to the tag.
2. Background of the Related Art
An RF tag system generally has a reader, also referred to as an interrogator or a base station, and tags that communicate with the reader using RF (radio frequency) signals. RF tag systems are shown, for instance, in U.S. Pat. Nos. 5,942,987 to Heinrich et al., 5,673,037 to Cesar et al., 5,550,547 to Chan et al., and 5,521,601 to Kandlur et al.
If the reader desires to read data from a specific tag, the reader will transmit a Group Select command addressed to all the tags, or a group of tags, requesting a response. The tags will each respond with its tag identification (ID), which identifies the specific tag to the reader. The reader then transmits a Read command having the tag ID of the desired tag. The specified tag will transmit a response signal, with the requested data. For a tag having a 40 Kbps data rate, it takes approximately 3.35 ms for the reader to transmit the Group Select command, 2.8 ms for the tag to issue a response, 3.15 ms to send the Read command, and another 2.8 ms for the tag to respond. Accordingly, the total transaction time is about 12.10 ms for a tag having a 40 Kbps data rate.
If the reader desires to write data to a specific tag, the reader will transmit a Group Select command addressed to all the tags, or a group of tags, requesting a response. The tags will each respond with its tag ID. The reader then transmits a Write Byte command having the desired tag ID and a first byte of data to be written to the tag. The tag acknowledges that it received a write data command and then writes the data to its memory (usually an EEPROM). The reader then sends a Verify Byte command, which contains the tag ID and the address location of the byte of data that is being verified. The tag responds by reading the byte of data that was written to the memory and sending a Verify Byte Response with that byte of data that was written to the memory. The process repeats for each byte of information to be written to the tag.
A typical write process is for eight bytes, which means that the Write Byte—Write Byte Response—Verify Byte—Verify Byte Response cycle is performed a total of eight times.
For a tag having a 40 Kbps data rate, it takes 3.35 ms for the Group Select command, 2.8 ms for the response, 3.35 ms for the Write Byte command, 1.4 ms for the Write Byte Response, 15 ms to write the byte to the tag, 3.15 ms for the Verify Byte command, and 1.4 ms for the Verify Byte Response. Accordingly, the total transaction time for an 8-byte write operation at a 40 Kbps data rate is 200.55 ms.